I suggest you buy two rubber balls, one about the size of a tennis ball, another about the size of a ping pong ball. The Tennis ball sized one should be soft enough that your fingers can squish into it, yet not completely squish it.

Tennis ball size one goes in the palm of the left hand palm up. Use all your fingers to squeeze the ball a few times to warm up. Now just use fingers 2 and 4, for a few reps then fingers 1 and 3. Use 1 finger at a time, then 3 at a time switching combinations. The point here is to use the little muscles in your fingers that rarely get used, let alone in the odd combinations.

With the left hand pointing up, put the ping pong sized ball between two fingers, Don’t Squeeze !!, rather roll the ball away and back towards yourself without letting it fall. Start using fingers 1 and 2, then use 1 and 3, then 1 and 4. Now fingers 2 and 3, then 2 and 4 etc…

The simple exercises will build both strength and dexterity at the same time and can be done while doing other things as long as your left hand is free. Like watching TV, taking a walk, stuck in traffic……